1. Technical Field
The present invention relates to a head-extended pile, and more particularly to a head-extended pile having an increased supporting force to support the load of a structure. Also, the present invention relates to a method for constructing such a head-extended pile with stability to proof stress of the pile, construction workability, and economic efficiency.
2. Background Art
When a building or a structure is built, construction of a foundation for reinforcing the ground is generally carried out according to conditions of the ground or the load of the building or the structure so that the building or the structure can be supported on the ground. Construction of a shallow foundation or a deep foundation is carried out based on various conditions, such as load of a structure. The shallow foundation is a foundation having a penetration width ratio below 1 while the deep foundation is a foundation having a penetration width ratio above 1. In the case of the shallow foundation, a structure is directly supported on the ground without using piles. In the case that a structure is not sufficiently supported on the ground, on the other hand, piles are used to reinforce a supporting force of the ground.
A pile foundation system is a foundation system characterized in that heads of the constructed piles are connected to a structure. Piles may be classified into steel piles, concrete piles, and composite piles on the basis of their material. Pile installation methods may be classified into pile driving, pile burying, and in-place casting.
The pile driving is a method characterized in that a pile is erected, and is then forcibly driven into the ground by hammering until the pile is fully penetrated into the ground. When the pile is forcibly penetrated into the ground by means of hammering energy, the pile is penetrated while the pile pushes through the soil around the pile. Consequently, a supporting force of the pile is excellent, and the pile is simply constructed.
However, the pile driving method has a drawback in that it is difficult to penetrate the pile vertically when the pile need to be deeply penetrated into the ground. Also, excessive vibration and noise are generated when the pile driving method is carried out. Consequently, the pile driving method is only limitedly used in urban areas due to certain restrictions.
On the other hand, the auger drilled piling is a method characterized in that a hole is previously bored in the ground, a cement paste is poured halfway into the hole, and then a pile is fixedly inserted into the hole. The auger drilled piling method can solve the drawbacks of the pile driving method. Currently, the pile burying method is mainly used to construct pile foundations in urban areas.
The construction of the foundation is extremely important in the aspect of building the structure. Various pile installation methods are used without consistency on the basis of the personal experience of constructors because conditions of the ground or vary according to site, or vary according to the site, or the operation of the pile-driving machine is not fully understood. As a result, the pile constructing work is not easily carried out.
FIG. 1 is a conceptual drawing showing the relation between an inherent proof stress of a pile and a constructional proof stress of the constructed pile. As shown in FIG. 1, load (PE) of a structure 11 is supported by means of constructional proof stress of a plurality of piles 12 penetrated into the ground below the structure 11. The constructional proof stress of the constructed pile 12 is the sum of a front end bearing force (TF) at the front end of the pile and a surrounding frictional force (SF) at the outer circumference of the pile. Generally, the inherent proof stress of the pile is larger than the constructional proof stress. However, the constructional proof stress of the pile is decreased due to its bad construction workability.
For example, a Φ400 high-strength concrete pile (hereinafter, referred to as “PHC pile”) has an inherent proof stress of 112 tf and a constructional proof stress of 60 to 80 tf. As a result, 32 to 52 tf of the inherent proof stress of the pile is wasted. Especially when auger drilled piling method is used, a drilled hole having a diameter larger than that of a pile is bored in the ground, the pile is penetrated into the hole, and cement paste is poured between the pile and the ground in order to increase a surrounding frictional force. However, a test of the pile after the construction is completed reveals that the surrounding frictional force is insignificant and most of the constructional proof stress is the front end bearing force. Consequently, it is necessary to increase the constructional proof stress near the inherent proof stress of the pile in the auger drilled piling method so that efficiency in use of the pile can be improved.
Therefore, the present invention has been made to solve the above problems, and it is an object of the present invention to provide a head-extended pile having an increased constructional proof stress so that the pile can have increased support load for a structure increasing the weight and the volume of the pile used in an auger drilled piling method. Moreover, the efficiency in use of the pile and economic efficiency of the pile are improved. It is also the object of the present invention to provide a method for constructing such a head-extended pile with stability to proof stress of the pile, construction workability, and economic efficiency.